Most laundry machine drain pumps use an AC induction motor or synchronous motor, operating off the mains frequency, which is typically 50 or 60 Hz. This fixed frequency, along with the mains voltage, determines the design of the pump, and hence the flow rate and maximum head height that can be obtained.
There are numerous disadvantages of a fixed mains-frequency pump motor. These are: (a) the starting torque is typically low, making the pump susceptible to being blocked, (b) separate pump designs are needed for countries having differing mains voltage and frequency, (c) the noise level during “ventilation” is high, particularly when running at a supply frequency of 60 Hz, (d) the torque varies with the mains voltage and (e) there is no feedback to indicate if the pump is faulty.
By the nature of the washing process debris, including strands of textile, is removed from the wash load and flushed into the sump of the washing machine. This debris is then pumped out along with the dirty wash water. Occasionally the pump impeller will catch on the debris, or will start accumulating textile strands and then stop pumping.
Problems of this nature have been solved in other industries such as sewage handling, by having powerful motors driving pumps, with large clearances and large flow passages. Home appliances, however, do not have the space available to utilise large flow passages. One small volume solution in use is the vortex impeller, where the impeller has a shroud around it that eliminates the jamming point between vane end and the ‘cutwater’. Unfortunately this pump has the disadvantage that it can still become blocked by bundles of fibres that become twisted “ropes” in the centre of the vortex.
Traditionally a laundry washing machine is turned off by employing one of two methods. In the first power to the washer is isolated using an expensive mains rated switch which de-energises it and makes power consumption negligible. The switch must be capable of carrying and breaking the total current required at the required voltage. In the second method the machine is made to appear it has been switched off by removing all power on indications on its control panel. In reality the machine's power usage is the same when it appears to be switched off as when it is switched on. These methods have the respective disadvantages that either a high current and/or voltage rated switch is required or power is consumed unnecessarily when the laundry machine is not in use.
The use of permanent magnet rotor three winding stator motors with electronic commutation of the windings is known. A common way of driving an electronically commutated three phase motor is to configure it in an H-Bridge between two dc rails, as shown in FIG. 5. The six power transistors Q1 to Q6 energise the motor windings by switching in a particular pattern. It is possible to have a high level of control over the speed and torque of the motor if feedback on the position of the rotor is available to the drive circuit logic which controls the switching pattern. The transistors are switched on by applying a voltage, usually 15V, from gate to source. Because the source voltage varies from 0V to the DC rail voltage, some technique is required to get the signal from the driving logic, which is referenced to 0V, to the high side gate which is referenced to the phase output (A, B, and C).
Common techniques are to use pulse transformers, purpose-designed ICs such as the IR2111, or optocouplers. A typical prior art circuit which employs an optocoupler for driving the high side transistor is shown in FIG. 6. When transistor Q2 switches off, noise is generated which tends to pull the optocoupler off. A logic inverter q4 ensures that turn off to the optocoupler means turn off of Q1. Since Q1 should always be off during turn off of Q2, correct operation of the circuit is not compromised. The disadvantage of this conventional optocoupler drive design is that high quality and therefore high cost devices are required.
During laundry machine spin dry cycles the spin tub is rotated at relatively high speed. At the termination of the spin cycle it is desirable to brake the motor to minimise spin tub run down speed. Conventionally this has been achieved by connecting resistors across the motor windings. This technique has the disadvantage that relatively high power resistors are required and these add to the cost of the laundry machine.